A battery consists of one or more cells. Over time the amount of charge that can be stored in a rechargeable battery decreases leading to shorter and shorter periods of battery effectiveness until the battery is able to store so little charge that it is deemed worthless and discarded.
Factors that act to reduce to amount of charge that can be stored in the cells of a rechargeable battery include the fact that the charging surface per unit area of a cell is never even nor of identical impedance resulting in uneven charge layering. Uneven charge layering results in charged “hot spots” across the charging surface. As the battery cycles through charging and discharging, the surface per unit area capacity imbalance increases with the inactive areas of the surface increasing until the cell or cells are deemed unserviceable.
A second factor that acts to reduce the amount of charge that can be stored in a rechargeable battery is the memory effect. This effect occurs when the battery is subjected to DC recharging before the battery has been fully discharged.
A third factor is the uneven charge layering caused by even DC charging which reduces the capacity of the cells.
A fourth factor that some battery designs suffer from e.g. Nickel Cadmium that also reduces the amount of charge that can be stored in the cells of a rechargeable battery, is dentrite growth. Dentrites are highly resistive filaments that “grow” from one plate to the other in the cell through the electrolyte separator. Dentrites effectively cause a resistance discharge within the cells. Dentrite growth is caused by even DC charging of the battery.